THE DEVELOPMENT OF SYMBRANCHUS MARMORATUS. 23 
that the part of the branchial rudiment not yet covered by the 
operculum is overlaid by the tissue masses of the body 
wall, what is equivalent to the posterior wall of the last cleft 
extending forwards so as to overlap the cleft rudiments and 
being continuous with the backwardly growing operculum. 
No space exists between the surrounding tissues and the three 
hinder arches, but between the backwardly growing oper- 
culum and the dorsal part of the first gill-arch a space is 
being hollowed out and lined by epithelium. The chinks in 
the solid cleft rudiments already described and this small 
space represent the opercular cavity at this stage. There is 
no communication between it and the exterior. 
The liver shows signs of histological differentiation, its cells 
no longer resembling those of the alimentary canal, but being 
distinguishable from the latter by their paler staining 
properties. Blood-corpuscles are plentiful in the developing 
liver. The spleen is bulky. 
Posteriorly the alimentary canal passes into a dilatation 
which receives the kidney ducts. This is the cloaca. 
In Stage 26 hyomandibular and hyoid arches are more 
clearly visible in sagittal sections. The gill-arches have 
increased in length, and are, therefore, more curved; the 
operculum has grown further back. There are now two small 
openings behind its posterior edge, which is at about the level 
of the second gill-arch, dorso-lateral in position, leading into 
the small differentiating opercular cavity. They each repre- 
sent a small dorsal portion of the lateral opercular opening 
such as is found in many teleosteans. Ventrally the cavity is 
not yet hollowed out. A transversely directed slit-like lumen 
commencing behind the level of the hyoid separates roof from 
floor in the developing pharynx. The oesophagus is solid, but 
the rest of the canal is gradually acquiring a lumen. 
The dorsal pancreas more or less surrounds the alimentary 
canal and one lobe has grown tail wards for a distance of 
180 ju (PI. 3, fig. 21). The spleen has now attained to a 
diameter greater that that of the stomach. The bile-duct and 
gall-bladder rudiments differentiate at this stage out of the 
